The aim of the proposed project is to partially characterize components of baroreflexes that differ strikingly in their effectiveness or potency. The baroreflexes to be investigated occur in snakes that provide unique model systems for the study of cardiovascular reflexes and hemodynamics. Terrestrial snakes that climb trees are subject to extreme gravitational disturbance of hydrostatic pressures and possess potent baroreflexes that assist the regulation of arterial pressure. In contrast, strictly aquatic species are never (voluntarily) subjected to gravitational disturbance of blood pressure; although baroreflexes are demonstrated to be present in these aquatic snakes, their actions are relatively feeble and arterial pressure is poorly controlled. The idea is to examine how various components of the baroreflex have been modified by natural selection in the cases of contrasting evolutionary history (semi-aquatic species provide an intermediate condition). To this end it is proposed to investigate: 1) sensitivity and response of cardiovascular effectors to pharmacological stimulation by principal autonomic drugs; 2) sensitivity of the complete baroreflex to alterations of arterial pressure; 3) localization of potential baroreceptor sites; and 4) cardiovascular actions of baroreflexes during gravitational disturbance induced by tilting, including assessments of changes in blood catecholamines, physiological and potential capabilities of cardiovascular effectors, changes in the distributional patterns of blood flow, and the dynamics of plasma filtration in the lung. These studies will eventually merge with ongoing investigations of baroreceptor characteristics on the afferent side of the baroreflex. The data will provide unique insights regarding the evolution of baroreflex. The data will provide unique insights regarding the evolution of baroreflex mechanisms and will enhance understanding of the specialized as well as general features of baroreceptor control systems. Appreciation of the evolutionary development of baroreflexes within a vertebrate subphylum should enhance our understanding of cardiovascular control generally, both in clinical applications and in our environment.